(i) Field of the Invention
This invention relates to a method of, and system for, storing heat. Primarily, it is directed to the storage of heat produced by off-peak electrical power. It is adaptable, however to store heat produced by other means.
(ii) Description of the Prior Art
Various methods and systems have been proposed for the storage of heat and consequently heat storage of itself is not new. It has been suggested that gravel, rocks, concrete, soapstone, and even blocks of steel and the like be heated to a high temperature and that the heat content of the material be thereafter used while the temperature of the heat absorbing means goes down. Such devices of the prior art, however, are limited in their applicability by the fact that the only heat stored is sensible heat, which is a function of the specific heat of the material used. Since the specific heat of available materials is low, usually in the neighbourhood of 0.2 B.T.U. per pound, the heat storage capacity of such material between, for example 200.degree. and 500.degree. F., is only 60 B.T.U. per pound. This renders such heat storing means impractical for space heating purposes because of the large bulk necessary to provide storage for large amounts of heat and because of the need to minimize heat loss from the heat storage means by insulation.
More practical heat storage systems have been devised in which heat of solution or heat of fusion, or a combination of both, are utilized. In such system, a crystalline material having a large amount of water of crystallization may be used, the material being so applied and selected that, upon being heated, the solid material melts or dissolves in its own water of crystallization, with the resulting storage of relatively large quantities of heat in the form of latent heat of fusion and solution. The heat so stored can be recovered by permitting the material to recrystallize.
U.S. Pat. No. 2,450,983 issued Oct. 12, 1948 to C. M. Osterheld provided an off-peak heat storage system using electric heaters which resided in the use of an automatic thermostatic control system to energize the electric heater in conjunction with an additional input as to whether it was night or day. An electric heater was provided for heating a heat storage mass. A light-sensitive cell was provided which was subject to daylight and to darkness. A plurality of thermostats were provided in a close heat-receiving relation to the heat storage mass and were adapted to move to closed position at different temperature values of the heat storage. A plurality of time delay relays were provided which were electrically connected in series electric circuit with certain of the thermostats, each relay being a normally open thermally-actuable member and a heating coil therefor. A switch which was controlled by the light-sensitive cell was provided for energizing the heating cells of time-delay relays after fall of darkness to cause closure of the thermally-actuable member after a predetermined period of time, and energizaion of such heater.
U.S. Pat. No. 2,677,243 issued May 4, 1954 to M. Telkes purported to provide an apparatus for storing heat, utilizing the principle of the heat of fusion, and a process for releasing the heat thus stored. According to that patentee, a limited portion of the heat storage composition was either maintained at, or was occasionally subjected to, a temperature substantially lower than the melting point of the composition, and specifically below the temperature of metastable supersaturation of the composition. By this means seeding nuclei were maintained or formed in a limited portion of the mass of heat storage material, and when the same had been undercooled below its melting point in an effort to extract heat therefrom, crystal formation based upon such nuclei spread rapidly throughout the mass, thus allegedly releasing large quantities of latent heat.
U.S. Pat. No. 2,856,506 issued Oct. 14, 1958 to M. Telkes purported to provide a method of storing and releasing heat utilizing a heat storage material and a method of heating a substance which included transferring stored heat thereto from such a material. The patentee provided a system, including apparatus and method, for storing heat at a relatively high temperature by utilizing as the heat storage medium, a crystalline solid which is dimorphic, that is to say, which changes from one crystalline form to another on the application of heat, which has a transition temperature between 300.degree. and 550.degree. F., and which has a relatively high heat of transition. The material suggested was anhydrous sodium sulfate, either by itself or modified by the addition of other salts, which could be converted by heating from the rhombic crystal form to a hexagonal form.
U.S. Pat. No. 3,382,917 issued May 14, 1968 to R. F. Rice provided a heat storing system including a unit in which heat may be stored at widely-varying temperatures, a heat exchanger, a chamber for heating fluid or other heater, a conduit leading from the storage unit to the exchanger through which fluid may be circulated to transfer heat from the unit to the exchanger, a second conduit leading from the exchanger to the heater for transferring heat from the exchanger to the heat, regulating means in the first conduit for varying the rate of heat transfer from the unit to the exchanger, and thermostatic means in the second conduit for controlling the regulating means. The regulating means suggested was a circulator and flow modifier. The fluid may be either gas or liquid, the circulator may comprise a blower, pump or fan, and the flow modifier may comprise a by-pass or throttle.
U.S. Pat. No. 3,989,927 issued Nov. 2, 1976 to G. O. Erb provided a storage heater for heating a gaseous heat extraction medium which is formed of a container of heat resistant material, preferably metal. At least one guide duct, in the form of a tube for carrying the gaseous heat extraction medium, extended through the container which held a heat storage medium in the form of a pourable bulk of particulate solid material. The thermal storage medium comprised a bulk in which the product of specific heat of the solid material and the bulk density of the mass was a specified amount.
In spite of these patents, there is still a need for a system having a high heat storage capacity, one in which there is substantially no heat loss from the system when heating is not required, one which does not require added exchangers in direct connection to the heat storage unit, and one which requires a fairly low charging temperature.
One system for the utilization of solar energy for cooling is described in an article by D. I. Tchernev entitled "Solar Energy Application of Natural Zeolites" U.S. N.T.I.S. pp Rep. 1977 pp 266055, GRI 1977 77(14)156. The author describes a system using natural chabozite on clinoptilolite as the solid absorber and water vapour as the working fluid in a zeolite system to provide domestic hot water and space heating. One such system combines a condensor and an evaporator into a single unit that is cooled by an external water loop. During the day, water vapour desorbed from the solar-heated zeolite is condensed in this unit and the liquid water is stored in the condenser in a condensate storage tank until evening. The heat of condenstion may be used for providing domestic hot water and for space heating. Whenever there is a demand for heat, hot water can be circulated through a coil located in air ducts of a corced air system, and the heated air is distributed throughout the building.